The invention relates to the manufacture of organic glasses based on polyurethane compositions of the type that are used to constitute optical lenses, more particularly, ophthalmic lenses, it being understood that urethanes have a formula resulting from the condensation of isocyanate groups with alcohol groups.
The invention aims essentially at developing a polyurethane composition able to polymerize into organic glasses, or transparent plastics materials, exhibiting a high refractive index, with low dispersion, as well as high mechanical strength, or more particularly high thermomechanical properties.
Considerable research has already been carried out with a view toward increasing the refractive index of transparent organic materials, in order to take better advantage of their light weight compared to inorganic glasses in the optical field. The research has led in particular to using sulfur-containing polyurethane resins.
Sulfur-containing polyurethane compositions can be obtained as described in U.S. Pat. No. 4,680,369 by reacting a polyisocyanate, such as xylylene diisocyanate or hexamethylene diisocyanate, with a polyol monomer including intermediate thio-ether chains, but this does not lead to a satisfactory refractive index. Others can be obtained by reacting a similar polyisocyanate with polythiol monomers as described in U.S. Pat. No. 4,689,387 and in European Patent Application 0 235 743. In that prior art however, monomers with complicated formulae which are difficult to use are derived from general types of monomers as currently used in the field or organic glass manufacture based on polyurethanes; and, when aliphatic compounds with mercapto groups --SH are used, they always include either thio-ether or ester groups in the monomer chain, as are obtained through polycondensation of glycols.
The polymers produced using these known techniques exhibit poor thermomechanical resistance, and in particular, their glass transition temperature is too low with respect to temperatures necessary for the surface treatment of optical lenses. Besides, technical and economic difficulties are encountered because these monomers correspond to compounds which are not easily available and which demand delicate synthesis procedures of, which renders them particularly expensive. In addition, they often cause other difficulties for the manufacture of polyurethanes, particularly when they form viscous solutions or when the reaction of the thiol functions on the isocyanate functions is disturbed by their structure.